The present invention relates in general to power supplies for electrical loads, and more particularly to a transformerless power supply of the electronic switching type that can be driven directly by alternating current from a commercial power line to energize a low voltage requirement direct current load.
Electronic, semiconductor-based circuits of, for example, the MOS-type or CMOS-type require only small amounts of low voltage direct current power to operate. To energize such circuits with power derived from commercial power lines of the alternating current type, requires the use of a direct current power supply interposed between the power lines and the electronic circuits. The direct current power supply converts the relatively high voltage alternating current from the commercial lines to a low voltage direct current that can be used by the associated electronic circuits. To date, such line driven power supplies have been primarily constituted by four well known types.
In the first type, a transformer is used to step down the line voltage and then rectify it to provide the required low voltage direct current. This first type of supply is disadvantageous due to the weight and bulk of its inductive element, i.e. the transformer.
In a second type of prior art supply, commonly referred to as a switching supply, a line driven rectifier charges a capacitor to the peak voltage of the alternating current line. A semiconductor switch operating at a predetermined duty cycle applies the capacitor voltage in a pulselike fashion to an output coil which in turn provides the required low voltage direct current to a load. Like the transformer-based first type of supply discussed above, this second type of supply suffers the same disadvantage of having a relatively heavy and bulky inductive element, i.e. the output coil.
The third and fourth types of known line driven direct current supplies are, respectively, resistive dividers and capacitive reactance dividers, both of which function in a well known manner to divide the alternating current line voltage into larger and smaller portions, the smaller portion of which is rectified to supply low voltage direct current to a load. Both of these divider type supplies are disadvantageous due to inefficiency and/or poor voltage regulation.
It should also be noted that thyristor devices can be directly driven by line voltage to provide variable direct current power to a load. However, the use of thyristors in a low voltage direct current supply is undesirable due to the lack of triggering stability at low switching voltages and due to other inherent disadvantages.
It is a purpose of the present invention to provide an inductorless, line driven, low voltage, direct current power supply for energizing a low power requirement direct current load. Such a power supply should be highly reliable, highly efficient, and low in cost.